Lord Rees of Ludlow keeps an open mind. He also takes the long view. He believes that humanity might one day colonise the galaxy but he thinks manned space flight now is a waste of money. He proselytises for science but he also gives human civilisation only a 50-50 chance of making it through the next 100 years, given the potentially lethal tools of science. He has spent his adult life trying to work out how the universe came to be and now thinks this universe could be just one of zillions.

He describes himself as a practising Christian who doesn't actually believe. He describes himself as old Labour but he doesn't sit with any party in the House of Lords. And he has just taken over as president of the oldest, greatest and most famous scientific academy on the planet: the Royal Society.

Martin Rees, 63, gets stuck into debate with a mix of seriousness and good humour that has made him a wow at literary festivals and a shoo-in as president at the Royal Society, the intellectual power base of the scientific revolution that began almost 350 years ago with Isaac Newton. He is also Professor Martin Rees, astronomer royal, and Sir Martin Rees, master of Trinity College at Cambridge, the college that was home to Francis Bacon, Isaac Newton, John Dryden, Lord Byron and - so far - 31 Nobel prize winners, but he also sees the cheerful side of life, death and creation.

He and other astrophysicists propose that although time, space and matter as we know them may indeed have begun roughly 13.7 billion years ago, there is no guarantee this universe will ever end.

All the stars and all the galaxies could fade and sputter, and drift ever further apart, frozen cinders trapped in an infinite expansion, in which any surviving life form would hibernate for ever longer, and develop ever more slowly. Time would drag, he would tell audiences, but there would still be time for every conceivable thought. At which point, he would impishly produce a Woody Allen quotation: "Eternity is very long, especially towards the end."

His most recent book argues that - what with a potentially devastating mix of nuclear weapons, global warming, genetic experiments, nihilistic terrorism, crashing asteroids and spooky experiments on the nature of matter itself - civilisation could destroy itself in the next few generations. But, he told the Hay literary festival this year, American readers were thought too impatient to be interested in Our Final Century. So his US publishers renamed the book Our Final Hour. His gift for being serious without being solemn is contagious: his colleagues at the Institute of Astronomy staged a conference in his honour and called it Making Light of Gravity.

He takes over as president of the Royal Society and the formal voice of British science from the Australian-born Sir Robert May, one of the more forceful figures of British science. "He has a very different style but I am sure he is the man for the time and his quiet, understated and reasoned approach will be well received and just as effective as Bob's," says another fellow of the Royal Society, plant scientist Christopher Leaver, of Oxford University.

Gerry Gilmore, of the Institute of Astronomy in Cambridge, sees Rees as an outstanding scientist whose real achievement may have been to make others think. "His special feature is not just his exceptional individual contributions, but his astonishing ability to know what is a really interesting question, and just as importantly, which questions are better avoided for the present."

Rees is lightly built and soft-spoken, with a stoop caused by a curvature of the spine that began in adolescence. He is the son of two Shropshire teachers, and although he reportedly took up mathematics because he was no good at languages, he remains one of science's great communicators. He is married to Caroline Humphrey, a Cambridge professor of anthropology with a lifelong interest in Mongolian nomads, Siberian shamans and Nepalese pastoralists.

He began his research in physics 40 years ago, just as astronomers confirmed that the universe must have had a beginning, and he, along with his Cambridge colleague Stephen Hawking and many others, began the heady process of calculating precisely how this universe must have taken shape. His intellectual interests range from the rarest and most violent beasts in the cosmos - gamma ray bursters and massive black holes - to the gentlest and most ubiquitous, gravity itself, but he also defends the idea of the "multiverse" in which this world is just one random experiment among many possible universes, co-existing but forever separate.

He will now go from enormous questions of space and time to detailed, partly political questions of energy policy, the climate debate and genetic research. The Royal Society is there to provide the government with best scientific advice possible at the time, and Martin Rees will be its voice. He once had to speak up for astronomy - now he must speak up for all science.

The CV

Born June 23 1942

Married 1986, Caroline Humphrey

Education Shrewsbury school; Trinity College, Cambridge (MA, PhD)

Career includes professor, University of Sussex 1972-73, Plumian professor of astronomy and experimental philosophy, University of Cambridge, 1973-91 Royal Society research professor 1992-2003, Astronomer Royal 1995-2005; president, Royal Astronomical Society 1992-94, British Association for the Advancement of Science 1994-95, Royal Society 2005-.